Optical packet switching seems to be the most promising way to take advantage of fibre bandwidth to increase router forwarding capacity, being able to achieve very high data rate operations. Optical flip-flops have been widely investigated mainly because they can be exploited in optical packet switches, where switching, routing and forwarding are directly carried out in the optical domain. Some known examples concern optical packet switches, where an optical flip-flop stores the switch control information and drives the switching operation. Former solutions for all-optical flip-flops exploit discrete devices or Erbium-doped fibre properties which suffer from slow switching times and high set/reset input powers. Several integrated or integrable solutions present a switching energy in the fJ range and switching times of tens of ps at the expenses of poor contrast ratios. On the other hand there is an integrated scheme exhibiting a very high contrast ratio value but with transition times in the ns range. In any case a trade off between contrast ratio and edges speed must be found as a function of the flip-flop application.
A known micro-resonators-based bi-stable element presents high optical operating power, pJ switching energies and microsecond switching times, theoretically reducible down to the order of tens of ps. Making a comparison with electronics, recent large-scale integration (LSI) circuits show switching energies of 1fJ although with slower switching speeds.
In H. J. S. Dorren, et al., “Optical packet switching and buffering by using all-optical signal processing methods”, J. Lightwave Technol., vol. 21, pp. 2-12, 2003, a solution based on coupled ring lasers is proposed. This solution offers a certain number of advantages: it can provide high contrast ratios between states; there is no difference in the mechanisms for switching from state 1 to state 2 and vice-versa, allowing symmetric set and reset operations; it presents a large input light wavelength range and a controllable switching threshold. Moreover, considering an integrated version of this kind of flip-flop, through numerical analysis a switching energy in fJ range has been demonstrated.
An optical flip-flop is a well known bi-stable circuit or circuit element capable of being stable in either one of two states, e.g. signifying a ‘1’ or a ‘0’, a ‘high’ state or a ‘low’ state, etc. A flip-flop can therefore represent or store a single bit of information. In an optical flip-flop, all signal processing is carried out in the optical domain—the input to the flip-flop is an optical signal, the output of the flip-flop is an optical signal and any intermediate signal processing by the flip-flop is in the optical domain. Similarly in an optical switch, all signal processing is carried out in the optical domain—the input to the switch is an optical signal, the output of the switch is an optical signal and any intermediate signal processing by the switch is in the optical domain.
Set-reset flip-flops (SR flip-flops) are also known. SR flip-flops usually comprise two inputs and if neither one is high, an output condition of the flip-flop is maintained in its current state (the status quo is maintained). If a first input is made high, while the other input is low, a particular output condition, e.g. output=‘1’ is provided, and if, instead, the second input is made high whilst the first input is kept low then the other output condition, i.e. output=‘0’ is provided. One of these conditions is the ‘set’ condition (e.g. the one which provides output=1), and the other condition, (i.e. output=0) is the ‘reset’ condition.
One known type of optical SR flip-flop is a coupled ring laser-based flip-flop. Y. Liu, et al., “Three-State All-Optical Memory Based on Coupled Ring Lasers”, IEEE Photon. Technol. Lett., vol. 15, no. 10, pp. 1461-1463, October 2003, discloses such an all-optical flip-flop arrangement.
As discussed, flip-flops are bi-stable elements. Multi-stable elements (for example elements or devices which have three stable states or more stable states are known. Investigation of such devices is found in articles such as:
“Three-state all-optical memory based on coupled ring lasers” Y. Liu; M. T. Hill; N. Calabretta; H. de Waardt; G. D. Khoe; H. J. S. Dorren; Photonics Technology Letters, IEEE Volume 15, Issue 10 Oct. 2003 Page(s):1461-1463; or
“Three-State Optical Memory Based on Coupled Ring Lasers” Jing Wang, Yuancheng Zhang, Antonio Malacarne, Antonella Bogoni, Luca Poti and Minyu Yao; OFC/NFOEC 2008, OMV5; or
“Multi-state optical memory based on serially interconnected lasers” Zhang, S.; Owens, D.; Liu, Y.; Hill, M. T.; Lenstra, D.; Tzanakaki, A.; Khoe, G. D.; Dorren, H. J. S.; 31st European Conference on Optical Communication, 2005. ECOC 2005. Volume 4, 25-29 Sep. 2005 Page(s):791-792 vol. 4.It is an aim of this invention to provide an improved flip-flop.